1. Field of the Invention
Firstly, the preset invention relates to a method and an apparatus for determining the ease of slip on road surfaces on running vehicles, that is, for detecting the friction coefficient of road surface. Secondly, the present invention relates to a method and a system for four-wheel steering of vehicles in which the steering of rear wheels is controlled by using the detected friction coefficient of road surface, when the front wheels of vehicle are steered.
2. Description of the Related Art
The method for detecting the friction coefficient of road surface and the method for four-wheel steering of vehicles have been disclosed, for example, in Unexamined Japanese Patent Publication No. 60-148769 (U.S. Pat. No. 4,964,481, DE 3,500,797, GB 2,153,311, FR 2,558,130). According to the former method of these two known methods, the friction coefficient of road surface is detected on the basis of the operating angle of steering wheel of vehicle, the vehicle speed, and the lateral acceleration acting on the vehicle body. According to the latter method, the steering angle of rear wheels on vehicle is corrected in accordance with the friction coefficient of road surface detected by the former method.
The above-described method for detecting the friction coefficient of road surface will be explained below in more detail. First, the slip angle of the front wheel is calculated from the operating angle of steering wheel and the vehicle speed. The slip angle correlates with the cornering force of front wheel, and the cornering force varies with the friction coefficient of road surface. Meanwhile, it is well known that the cornering force is proportional to the lateral acceleration acting on the vehicle body. If the operating angle, the vehicle speed, and the lateral acceleration are detected, therefore, the friction coefficient of road surface can be calculated on the basis of the detection results.
The lateral acceleration which the vehicle body is to be subjected during the turning of vehicle actually acts on the body only when some time elapses after the start of turning. Since the lateral acceleration acts on the vehicle body subsequent to a delay after the start of turning of the vehicle, there is a delay in detecting the lateral acceleration of vehicle body. For this reason, the friction coefficient of road surface cannot be calculated quickly by the above-described method.
The cornering force calculated from the lateral acceleration acting on the vehicle body is equivalent to the sum of the cornering forces generated on the front and rear wheels. When the friction coefficient of road surface is calculated on the basis of the cornering force obtained from the lateral acceleration, therefore, the slip angle of not only the front wheels but also the rear wheels should be taken into consideration. With the conventional method, in which the slip angle of front wheels only is taken into consideration, the friction coefficient of road surface cannot be exactly detected, resulting in poor accuracy of the detected friction coefficient of road surface.